1. Field of the Invention
This invention relates to a display device having a plurality of display pixels arrayed in a matrix form to display an image, to a driving method thereof and, for example, to a self-luminous display device in which each display pixel is configured by a self-luminous element such as an organic EL (Electro Luminescence) element.
2. Description of the Related Art
In recent years, much attention has been focused on organic EL display devices as monitor displays for portable information terminals since the devices have such characteristics as lightness, thinness, and high luminance. A typical organic EL display device includes organic EL elements as self-luminous elements incorporated in display pixels which are arrayed in a matrix form to display an image. In this organic EL display device, a plurality of scanning lines are disposed along rows of the display pixels, a plurality of signal lines are disposed along columns of the display pixels, and a plurality of pixel switches are disposed near intersections of the scanning and signal lines.
Each display pixel includes a pixel switch, driving element and organic EL element. The pixel switch is connected to receive a video signal from a corresponding signal lines in response to a scanning signal from a corresponding scanning line. The driving element is connected in series with the organic EL element between a pair of power lines to supply a driving current corresponding to the video signal from the pixel switch. The driving element and pixel switch are formed of thin-film transistors disposed on a glass or synthetic resin substrate, a conductive substrate, or a semiconductor substrate having an insulating film of SiO2 or SiN, for example.
The organic EL element has a structure in which a luminous layer is formed of a thin film containing fluorescent organic compounds of red, green or blue, and is held between the cathode and the anode so that holes and electrons are supplied and recombined in the luminous layer to produce excitons. The organic EL element outputs light radiated upon deactivation of the excitons. The anode is a transparent electrode formed of ITO or the like and the cathode is a reflective electrode formed of a metal such as aluminum. With the this structure, the organic EL element can provide a luminance of about 100 to 100000 cd/m2 with an applied voltage of just 10 V or less.
The driving current of the organic EL element is controlled by utilizing the constant current characteristic of a driving thin-film transistor serving as the driving element. FIG. 28 shows the relation between the gate-source voltage Vgs and the driving current I1 of the driving thin-film transistor. If the voltage Vgs varies, the current I1 flowing at this time is determined according to equi-Vgs lines as shown in FIG. 28. However, while the transistor is operated in a saturation region, the current I1 can be kept substantially constant even if the drain-source voltage Vds varies. In the I-V characteristic of the organic EL element shown in FIG. 28, if the voltage Vds is determined at a certain value, the intersection between the equi-Vgs line and the I-V characteristic curve becomes an operating point of the organic EL element and the current I1 flows when voltage V1 is applied to the organic EL element. The current-luminance characteristic of the organic EL element is approximately linear, and if the current is constant, the luminance is also constant. Therefore, even if the I-V characteristic varies, the current or luminance is kept constant in so far as the transistor characteristic is kept unchanged.
Further, if the preset potential from the signal line X is applied to the gate of the driving thin-film transistor as the voltage Vgs by turning ON the pixel switch, the operating point of the organic EL element which is an intersection between the I-V characteristic curve and the equi-Vgs line of FIG. 28 can be selected so that multi-gradation display can be attained.
In a normal liquid crystal display device, the brightness of the backlight is generally adjusted to optimize the power consumption and ease of observation of an image depending on the service environment. For example, when the user carries around a portable information terminal which is battery-driven, electricity of the battery is saved by causing the user to select a low-power consumption operation in which the backlight is made dark or automatically changing the operation mode into the above operation when it is battery-driven. The brightness of the backlight can be made dark by lowering the power-supply voltage applied from the exterior.
On the other hand, the organic EL element is a self-luminous element whose luminance depends on a driving current thereof. Therefore, the luminance of the organic EL element cannot be adjusted by changing the power-supply voltage.
In a gradation display system in which a driving thin-film transistor turned ON/OFF in the non-saturation region is used, it is considered to adjust the ON-time of the thin-film transistor in order to attain desired luminance and gradation. However, extremely slight time adjustment is required and, as a result, it becomes difficult to adequately set either the luminance or gradation.
Further, it has been considered to change the video signal level in order to attain desired luminance which is half the maximum luminance, for example. However, if the currents flowing through all of the organic EL elements in the luminance adjusting system are equally reduced, the white balance cannot be maintained due to a difference in the luminance characteristics of the organic EL elements which depend on the luminescent colors of red, green and blue. If a correction circuit which corrects variation amounts of video signal levels for respective luminescent colors is used in order to solve the above problem, it cannot be avoided that the circuit configuration is complicated in comparison with that of the brightness adjusting system of the liquid crystal display device.